v2.81 Rock generator
The answer below is obsolete since the Rock Generator is now part of Extra Objects add-on since commit rBA7347930a0fa14e752929e28c9b48adf74597b018. The separate Rock Generator add-on has been removed from the add-ons contrib repository by commit rBAC70b649775eeeebedb02c1c7b7aa996a7f6294177.



The release versions only contain the official add-ons. Anything that is from the Blender Add-ons Contrib repository isn't included.

You can either clone the repository if you're familiar with git or you can download a .tar.gz archive of the latest commit of the contribution add-ons using the following link. Extract and copy the files into ./2.81/scripts/addons_contrib/ and you should be able to activate them.